In the field of optical communications, it is well known to use semi-conductor laser diodes to generate a narrowband optical signal onto which data is modulated for transmission through an optical medium such as an optical fiber link. In order to obtain desired characteristics of the optical signal (such as center wavelength, line width, signal reach, for example) the output power of the laser diode must be maintained within narrow tolerances. Because different laser diodes have different output power characteristics in response to a given driving current, it is desirable to monitor the output power from each laser diode, and adjust the driving current as needed to maintain the output power at a desired level. FIG. 1 schematically illustrates a typical laser control system 2 for this purpose.
In the laser control system 2 of FIG. 1, a typical Transmission Optical Sub-Assembly (TOSA) 4 comprises a semiconductor laser diode 6 and a Back facet Monitoring (BFM) photodetector 8 mounted on a substrate10 such as a printed circuit (PC) board. A controller unit 12 supplies a bias current IBIAS 14 and a modulation current IMOD 16 to the laser diode 6 to generate an output optical signal 18 for transmission. In order to control the laser diode, the controller unit 12 receives a back facet monitoring (BFM) current IBFM 20 from the photodetector 8, and a temperature indication (TPCB) 22 from a sensor (not shown) mounted on the substrate proximal the laser diode 6. Ideally, the temperature sensor would measure the temperature of the laser diode 6 itself. However, since this is often impractical for various reasons, the temperature sensor is typically mounted to detect the temperature of the substrate (or printed circuit board) near the laser diode 6. Since the thermal properties of the substrate are known, or at least known to be approximately constant within the operating temperature range of the laser diode 6, then the substrate temperature (TPCB) can be used as a proxy for the actual laser temperature.
Typically, the controller 12 is coupled to a memory 24, which includes a non-volatile memory 26.
In normal operation, the controller 12 adjusts the bias current IBIAS 14 so as to maintain the BFM current IBFM 20 at a predetermined value. In some cases, the controller 12 may also adjust the modulation limits of modulation current IMOD 16 to maintain a desired extinction ratio. In TOSAs in which the laser diode 6 is a bulk semiconductor laser diode, this operation allows accurate control of the power level of the output optical signal 18. However, this operation generally will not work for injection seeded lasers, such as an injection seeded Fabry-Perot laser. As may be seen in FIG. 1, in the case of an injection seeded laser, a seed light 28 is provided to the TOSA 4, and injected into the cavity (not shown) of the laser 6. In this case, the BFM current IBFM 20 will contain a component proportional to the injection power of the seed light 28. This raises a difficulty in that the injection power level of the seed light 28 is unknown, and may change rapidly with time. Under these conditions, the BFM current IBFM 20 does not provide a accurate indication of the power level of the optical signal 18, and thus the conventional method of controlling the output power based on the BFM current IBFM 20 cannot be used.
In order to address this problem, injection seeded lasers are typically controlled using the temperature TPCB 22. In this case, the laser control function is based on the assumption that the power level of the output optical signal 18 is proportional to the laser temperature. However, in fact, the correlation between temperature and output power is poor, and can change with changing operating conditions (e.g. seed injection power) and laser aging. As a result, temperature-based control methods tend to be significantly less accurate than methods based on the BFM current 20.
Techniques that overcome the above-noted limitations in the prior art remain highly desirable.